There are two different reason for choosing one over the other:
The eye's sensitivity peaks in the green - so less light intensity gives more "vision".
But: Red doesn't damage the night adaptation, so doesn't need much illumination anyway.
Balance this against the film's sensitivity, and sometimes green is better, sometimes red. I use red with the slower EFKE films, green with most others. Sometimes I just accept that my darkroom isn't very light-proof, and after five minutes in there I can see what I'm doing just by the light from the light leaks. Many people would be very surprised at just how much light you can have and still not fog the film...